In pictures: Boston Marathon explosions

Two deadly bomb explosions strike at the finish line of the annual Boston Marathon.

The FBI is leading the investigation into Monday's twin blasts at the Boston Marathon. Special Agent Richard DesLauriers described it is as "potential terrorist investigation".

At least three people died and more than 100 were injured - some critically - in the explosions close to the finish line of the annual marathon.

This image from NBC shows the first blast, with runners and spectators approaching the finish line.

The second explosion came as people rushed to help those caught in the first.

A photographer captured the second explosion as runners made their way to the finishing line.

Runner Mike Mitchell of Vancouver, Canada, said he had just finished the race when he looked back at the finish line and saw a "massive explosion".

The incident came at around 14:50 local time (18:50 GMT) - about three hours after the winners crossed the line.

Some were able to walk away from the scene of the explosions.

What had been a scene of celebration was suddenly shattered. Runners were left reeling in shock and despair.

Those seriously injured in the explosions were quickly taken to hospital, leaving a scene of carnage behind.

The cause of the explosion has not been confirmed but the White House says it is in contact with state and local authorities. Emergency services are on high alert and state police have been deployed at the scene.

Speaking at the White House, President Barack Obama said any group responsible would feel the "full weight of justice".